This invention relates to combustors and more particularly to a swirl-stabilized combustor operating in a lean-premixed mode that mitigates high-amplitude, discrete-frequency, self-sustaining pressure oscillations within a certain window of operating conditions.
Combustion technologies remain one of the most power-dense sources of energy. Even with the emergence of alternative energy sources, combustion is necessary to keep up with energy demands, and irreplaceable within some domains of application.
The chemical products of combustion are often undesirable, being implicated in everything from local health and environmental damage to global climate change. One of the major classes of pollutants emitted by combustors is nitric oxide (NO and NO2, collectively known as NOx). NOx can be formed thermally as otherwise inert atmospheric N2 reacts at high temperatures. Lean-premixed combustion has been shown to substantially reduce NOx emissions from conventional non-premixed combustion by improving mixing and lowering the flame temperature. Lean-premixed combustion is very susceptible to combustion instability, however, which is a coupling between the acoustics, flowfield and reaction zones within the combustor that lead to strong pressure oscillations.
The acoustic oscillations in these combustors can reach unacceptably high levels. While such oscillations can, in principle, run afoul of noise pollution regulations, they also pose a danger to the operation of combustors in the premixed mode. Strong, discrete pressure oscillations are capable of causing mechanical damage to the combustor or nearby machinery. These pressure oscillations are accompanied by strong oscillations in the flow velocity, which can grow to sufficiently large amplitudes to cause flame blow-off or flashback.
Increasing focus on producing power with lower emissions has been a driving force in the study of the dynamics of lean premixed combustion systems. Work has progressed along two tracks: understanding the underlying physical mechanisms and developing strategies and models that can be used to suppress combustion instability and make lean premixed combustors practical devices [1]. Numbers in brackets refer to the references appended hereto. The contents of all of these references are incorporated herein by reference.
In a non-reacting domain, microjets have found use in the control of supersonic jet instabilities [2,3]. Air injection is accomplished through ports (“microjets”) whose area is small compared to that of the main jet. In this context, the microjets are used to alter the flow field, but are not used to add longitudinal angular momentum to the flow.
Microjets have also found application in the suppression of combustion instability for non-swirling flows. Initial application of microjet injectors for combustion instability looked at the injection of fuel into the flow [4].
The injection of air through microjets in the streamwise direction (“axial” injection) and in the cross-stream direction (“normal” injection) have both demonstrated the capability of reducing or suppressing combustion instability in a backward-facing step combustor under various conditions [5, 6, 7]. By injecting air, rather than fuel, through the microjets, the creation of locally fuel-rich fuel/air mixtures is avoided.
The backward-facing step combustor features a rectangular cross section and a discontinuous increase in the area of the channel in which combustion takes place (the backward-facing step). This step allows the flame to anchor, but also provides a geometrically favorable location for vortex shedding to occur. Similar area expansions are commonly used in practical combustors as a means of controlling where the flames anchor. Axial microjets were located on the downstream face of the step, injecting air toward the exhaust. Normal microjets were located on the upper face of the step, injecting air across the mean flow immediately before the expansion. The success of these microjets in reducing the amplitude of certain frequencies of instability at different operating equivalence ratios (see the cited references for details) provided the initial inspiration for the use of microjets in the swirl-stabilized combustor.
Researchers have explored methods for mitigating these self-sustaining pressure oscillations. Arakeri, et al. explored the use of microjets in a high-velocity jet for the reduction of turbulent noise intensity [8]. In Arakeri, microjets were mounted at an angle to the flow in the plane formed by the radius and longitudinal axis. The microjets were angled so that the flow coming out of the jets had velocity components in the streamwise and inward radial directions. These microjets did not introduce swirl in the flow. The authors were able to show a reduction of 2 dB in intensity of the turbulent noise.
Kumar, et al. [9] studied the use of microjets to stabilize high-speed impinging jets, particularly as they are applied to vertical take-off and landing aircraft. These researchers were able to reduce the noise by 15 to 20 dB, and improve the lift-to-drag characteristics of the impinging jet. Microjets were aimed in the streamwise direction, slightly angled in the radial direction. They did not introduce swirl into the flow.
Altay, et al. [7] explored the use of microjet actuation for suppressing combustion instability in a backward-facing step combustor. Their results showed that microjet injection of air into the flame anchoring zone provided a simple means of reducing the intensity of combustion instability in certain operating regimes. They examined the use of streamwise and perpendicular microjets.
Zhuang, et al. [10] investigated microjet injection for suppression of instabilities in a supersonic cavity flow. Microjets aimed normal to the mean direction of the supersonic flow were located on the outside upper lip of a cavity. Microjet actuation produced a 20 dB reduction in sound pressure level under specific conditions.
It is an object of the present invention to provide a combustor that mitigates high-amplitude, discrete-frequency, self-sustaining pressure oscillations capable of reducing overall sound pressure levels by up to 17 dB within a certain window of operation conditions.